Re: [PATCH] mm: make VM_MAX_READAHEAD configurable

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 9 Oct 2009 14:29:52 +0200
Jens Axboe <jens.axboe@oracle.com> wrote:

> On Fri, Oct 09 2009, Peter Zijlstra wrote:
> > On Fri, 2009-10-09 at 13:19 +0200, Ehrhardt Christian wrote:
> > > From: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
> > > 
> > > On one hand the define VM_MAX_READAHEAD in include/linux/mm.h is just a default
> > > and can be configured per block device queue.
> > > On the other hand a lot of admins do not use it, therefore it is reasonable to
> > > set a wise default.
> > > 
> > > This path allows to configure the value via Kconfig mechanisms and therefore
> > > allow the assignment of different defaults dependent on other Kconfig symbols.
> > > 
> > > Using this, the patch increases the default max readahead for s390 improving
> > > sequential throughput in a lot of scenarios with almost no drawbacks (only
> > > theoretical workloads with a lot concurrent sequential read patterns on a very
> > > low memory system suffer due to page cache trashing as expected).
> > 
> > Why can't this be solved in userspace?
> > 
> > Also, can't we simply raise this number if appropriate? Wu did some
> > read-ahead trashing detection bits a long while back which should scale
> > the read-ahead window back when we're low on memory, not sure that ever
> > made it in, but that sounds like a better option than having different
> > magic numbers for each platform.
> 
> Agree, making this a config option (and even defaulting to a different
> number because of an arch setting) is crazy.

Given the (increasing) level of disparity between different kinds of
storage devices, having _any_ default is crazy.

Would be better to make some sort of vaguely informed guess at
runtime, based upon the characteristics of the device.